Science Buddies: "Ask an Expert"

I am doing a science fair project called "Which orange juice has more vitamin C, fresh squeezed, carton, or frozen concentrate?" I found the idea on Science Buddies and ordered all the supplies to conduct the experiment. I cannot seem to get past the initial titration process with vitamin C tablets. I never could get a color change. I switched the measurements and used 20 mL of starch and 10 drops of vitamin C an then added my iodine an i got dramatic color changes, until we mixed it then it went back to clear its original color. It's supposed to stay that blue-black color the whole time was my understanding. What do i need to do to get this titration process to work right?

Hi,

It is fairly obvious you have not added enough iodine to fully oxidize the vitamin C. You are correct that at the end point of the titration the solution should turn blue-violet and stay that way. You might start by adding just one drop of the vitamin C to your starch solution and titrating that. From the results of titrating that you can judge how many drops of vitamin C to add to require a reasonable amount of iodine solution to reach an end point.

Good luck on the experiment!

Best regards,

Barrett L. Tomlinson